An American warplane shot down a Syrian government jet on Sunday, the Pentagon said, marking the first time in Syria’s civil war that a U.S. pilot has struck a regime plane and signaling an increased willingness by the Trump administration to directly challenge President Bashar al-Assad and his allies.

On Sunday, the U.S. military said it had shot down the Syrian SU-22 after regime forces twice attacked members of American-backed Syrian fighters leading the assault on Raqqa, the self-declared capital of the Islamic State terror...